The chlorination and beneficiation of titaniferous ores by chloride processes produce iron chlorides as a by-product. The chlorine gas which is used to effect the chlorination may be recovered from the ferrous chloride by oxidation to chlorine gas which can be recycled back to the chlorinator or beneficiator reactors.
Where the titaniferous ore is chlorinated only to the extent necessary to remove iron as iron chloride (ferrous and/or ferric) and the titanium dioxide remains substantially unchanged, this process is called "beneficiation". Where the titaniferous ore is chlorinated under conditions sufficient to yield titanium tetrachloride as an end product, this process is understood as a "direct" chlorination process leading ultimately to the production of titanium dioxide. In either case, ferrous and/or ferric chloride are produced. The chlorinating agent in either case is elemental chlorine.
When the ferrous chloride (FeCl.sub.2)is condensed and separated from the off-gas of an entrained flow chlorinator, a chlorinator, or beneficiator, only a small amount of unburned carbon remains with the solid ferrous chlorides. If an indirect heat transfer means is used for separating the chlorine, plugging by iron chlorides can result. Accordingly, all heating and cooling are by a direct method.
In the process of the present invention, there is no special preparation of the fluid bed medium. This medium is formed of a particulate solid material which is inert to chlorine and ferrous chloride. A most convenient material is ordinary silica sand. Thus, the instant process avoids the high costs of material preparation and also stabilizes the reaction rate because of the consistency of the physical and chemical properties in a fluidized bed medium.
Problems are inherent in the oxidation of FeCl.sub.2 or FeCl.sub.3 to elemental chlorine. The main problem with the full oxidation of FeCl.sub.2 or FeCl.sub.3 to Cl.sub.2 is that at low temperatures where the thermodynamics are favorable, the reaction is slow. At higher temperatures where the reaction proceeds at a practical rate, the thermodynamics are unfavorable and the reaction is far from complete.
Efforts to overcome this problem are disclosed by Dunn in U.S. Pat. Nos. 3,887,694 and 3,376,112; and in Bonsack U.S. Pat. Nos. 3,944,647 and 3,919,400. These patents teach the use of catalysts to speed up the reaction at lower temperatures where the thermodynamics are more favorable. Dunn in U.S. Pat. No. 3,865,920 and Bonsack in U.S. Pat. No. 4,094,854 also suggest systems operating at higher temperatures where unreacted FeCl.sub.3 is separated and recycled back to the oxidation zone. Dunn in U.S. Pat. No. 3,865,920 also suggests the use of a very long "fuel pipe" in the oxidation zone discharge which pipe is held at a lower temperature.
Another severe problem with FeCl.sub.2 or FeCl.sub.3 oxidation to elemental chlorine is the formation of hard, dense Fe.sub.2 O.sub.3 deposits on the inner walls especially near the oxidation zone discharge. Attempts to solve this problem were the subjects of Sawyer U.S. Pat. No. 2,642,339; Nelson Nos. 3,050,365 and 3,092,456; Reeves No. 3,793,444; and Mitsubishi No. 4,073,874.
Nelson, in No. 3,092,456 introduces carbon in the discharge line of the oxidizer. In Nelson's process the reaction is essentially complete. Moreover, Nelson is oxidizing iron chloride to chlorine in a gas-gas reaction rather than in a gas-solid reaction as used in the present invention.
U.S. Pat. No. 4,055,621 to Okudaira teaches a process for obtaining chlorine from iron chloride. Iron oxide is added to the iron chloride which is prepared by chlorinating iron-containing titanium ore, in an amount above 10% by weight of the resulting mixture and charging the mixture in solid phase into a fluidizing roasting furnace for oxidation. Any overflow is oxidized in a second reactor. The iron oxide thus obtained is recycled to the primary reactor for controlling the reaction temperature in the furnace.
U.S. Pat. No. 4,140,746 to Turner et al relates to the recovery of chlorine values from iron chloride produced from the chlorination of titaniferous material containing iron and particularly from the carbo-chlorination of ilmenite which, for example, can be the first stage in the so-called chloride route to form titanium dioxide pigment. The iron chloride which may be ferric chloride or ferrous chloride is subjected to a combination of reduction and oxidation reactions. In the reduction reaction, ferric chloride is dechlorinated to ferrous chloride by a reducing agent suitable for producing a chloride compound for recycling to the chlorination process. In the oxidation reaction ferrous chloride is oxidized to ferric oxide and ferric chloride. The ferric chloride is recycled to the reduction reaction. According to Turner, by this method the chlorine values are recovered from the by-product iron chloride by a route which avoids the difficult reaction between ferric chloride and oxygen to produce chlorine and ferric oxide.
U.S. Pat. Nos. 4,174,381 to Reeves et al teaches an improved process and an apparatus for producing chlorine and iron oxide in a multistage recirculating fluidized bed reactor wherein ferric chloride in the vapor phase is reacted with an excess of oxygen at temperatures from 550.degree. to 800.degree. C. The improvement comprises utilizing a reactor that includes an initial "dense" zone and a downstream "dilute zone". In the dense zone, a fuel is burned, reactants and recirculated iron oxide particles are heated, ferric chloride is vaporized and at least 50% of the ferric chloride is converted to chlorine and iron oxide. In the downstream dilute zone, the conversion of ferric chloride is continued to greater than 95% completion.
European Patent publication No. 5054 discloses a process for the preparation of micaceous iron oxide which comprises reacting ferrous chloride substantially free from disruptive impurities, such as carbon, with oxygen at a temperature of 300.degree. to 1200.degree. C. The process can be carried out in a fluidized bed and it can form a part of a process for the recovery of chlorine values from iron chloride. The presence of carbon gives a nonmicaceous iron oxide. Also, a fluid bed reactor has disadvantages in that Fe.sub.2 O.sub.3 cannot be discharged continuously as in a packed bed system, without entraining unreacted FeCl.sub.2. This is because a fluid bed regime is a perfectly mixed reactor which makes separation of fresh feed and product impossible.
Another reference which relates to the recovery of iron oxide and chlorine by oxidation of iron chlorides is the patent to Hartmann U.S. Pat. No. 4,060,584. The invention described therein is directed specifically to the oxidation of chlorinator dust which is essentially ferrous chloride contaminated with coke and other metal chlorides and oxides. The amount of coke present in the chlorinator dust derived from the chlorination of ilmenite is about 23% to 25% by weight. Before the treatment in the oxidation reaction reactor, the temperature is elevated to 700.degree. C. by burning carbon monoxide in oxygen and then the chlorinator dust is added. During reaction the temperature reaches 750.degree. C., and afterwards the solid/gas mixture is separated in a cyclone separator. The resulting solid mixture showed a coke content of 53.8%. Hartmann states of his process at column 3, line 24, "Since the reaction is carried out at relatively low temperatures, i.e., 500.degree. to 800.degree. C., combustible secondary constituents of the chlorinator dust, such as carbon, are definitely not oxidized". This avoids sintering of the iron oxide and enables easy control of the reaction. Hartmann describes a total chlorination procedure instead of a selective chlorination procedure. In a selective procedure the amount of carbon present in the reaction mass is much less than in chlorinator dust from a total chlorination process.
In a beneficiation process taught by V. G. Neurrgandnkar, Journal Chem. Tech. Biotechnol, 1986, 36, 27-30, the carbon to ilmenite ratio is 8%. In a total chlorination reaction the ratio is 20-30%. Thus, the carbon content in the off-gas in a beneficiation reaction is much lower than the carbon content in a total chlorination reaction. This is also true in an entrained flow chlorination such as that taught by Bonsack, supra. The carbon used in this case is reactive and thus much easier to burn and no carbon is left over. As described in "Mitsubishi Process for Upgrading Ilmenite and Chlorination Recirculation", S. Fukushima and Y. Sugawara, p. 453, the ratio of carbon to ilmenite is 8% in such a reaction.
Other references relating to beneficiation and entrained flow chlorination teach carbon/ilmenite ratios of less than 20%. These include U.S. Pat. No. 4,343,775 and U.S. Pat. No. 4,094,954 to Bonsack; U.S. Pat. No. 4,389,391 to Dunn; U.S. Pat. No. 4,046,853 to Robinson; and a total chlorination process by described by Robinson in U.S. Pat. No. 4,624,843 which teaches about 7.5%, but preferably less than 12.5% carbon/ilmenite ratio.
As taught in the references cited above, the carbon content in condensed iron chloride is much lower in beneficiation processes than in total chlorination processes because beneficiation is usually run at a carbon deficiency condition. The carbon content in iron chloride condensed from entrained flow chlorination is much lower than that resulting from conventional chlorination processes which yield chlorinator dust, because the highly reactive carbon tends to be burned out entirely. Entrained flow chlorination yields only "left over" carbon, not blowover carbon as in a fluid bed reactor. The carbon/ilmenite ratio is lower in an entrained flow chlorinator.
In the case of condensed ferrous chloride from a total chlorinator, e.g., a fluidized bed chlorinator, the carbon content must be reduced from the normal 20 to 25% by weight. The carbon, together with any ore in the off-gas stream can be separated by conventional separation methods, such as using a hot cyclone separator, electrostatic precipitator (U.S. Pat. No. 4,094,954) or a "knock out pot". This removal can be effected at temperatures above the melting point of ferrous chloride (674.degree. C.). The carbon and ore solids can be most efficiently removed from a hot gas stream by using a ceramic fiber bag in a baghouse. The temperature can be as high as 800.degree. C. The carbon content in the condensed FeCl.sub.2 under these conditions is below about 12% and usually, the removal of carbon and ore is almost complete. If the collection is good, the carbon and ore are isolated with good separation before the ferrous chloride condenses.
The carbon content in the condensed ferrous chloride solid is then burned with oxygen to supply heat required to keep the fluidized bed medium hot, and to supply heat lost through the wall of the reactor. The presence of too much carbon in the ferrous chloride-containing mixture from a total chlorinator is undesirable because it consumes too much oxygen in burning and forms too high a concentration of carbon dioxide. This reduces the chlorine concentration. If chlorine containing excessive CO.sub.2 is returned to the fluidized bed chlorinator, carbon needed for the reaction is consumed because under these conditions, carbon dioxide in the recycle gas reacts with carbon to form carbon monoxide. The whole process is, therefore, rendered less efficient and hence it is preferred practice to lower the carbon dioxide content in the recycle gas by keeping the carbon content of the condensed ferrous chloride/carbon mixture preferably less than 12%.
As can be seen from the prior art above, in various methods for chlorinating titaniferous materials, e.g., ilmenite, rutile, and titaniferous slags, to produce TiCl.sub.4 and iron chlorides, chlorine is generally the chlorinating agent, and chlorine is recovered from iron chlorides by oxidation to Cl.sub.2 and Fe.sub.2 O.sub.3.